This invention relates to a method for regulating concentration and distribution of oxygen in Czochralski drawn silicon crystal rods through variation of both the magnitude and relative sense of direction of seed and crucible rotation rates. In another aspect, the invention relates to a method for achieving uniform axial and radial distribution of oxygen in Czochralski grown silicon rods through increasing crucible rotation rate to pre-selected values as a function of crystal rod growth and melt consumption while rotating the crystal in a direction opposite to that of the crucible and at a rate higher than that of the crucible. In yet another aspect, the invention relates to a method for introducing various levels of oxygen into the Czochralski grown silicon rod through choice of initial rotation magnitude and direction of the seed and crucible.
The production of single crystals from materials such as silicon plays an important role in semiconductor technologies. A suitable method for growing the silicon crystals is known as the Czochralski technique in which a seed crystal, which has the desired crystal orientation, is introduced to a melt of silicon. The silicon melt can also contain certain dopants which are introduced for the purpose of modifying the electrical characteristics of the silicon as is known in the art. The melt is contained in a silica crucible or vessel which is heated so that the silicon melt is at, or slightly above, its melting point. The seed crystal is slowly withdrawn from the melt, in an inert atmosphere such as argon, and the silicon solidifies on the seed to produce the growth of a crystal rod. Generally, the Czochralski technique is utilized for producing single silicon crystal which is utilized by the electronics industry. A cylindrical crystal is produced by rotating the crystal at a constant rate as it is drawn. The crucible is conventionally rotated at a constant rate in the opposite direction for the purposes of assuring thermal symmetry in the growth environment. The withdrawing rate and power to the heating means is first adjusted to cause a neck down of the crystal. This eliminates dislocations caused by the thermal shock which occurs when the seed crystal initially contacts the melt. The withdrawing rate and the power is then adjusted to cause the diameter of the crystal to increase in a cone shaped manner until the desired crystal diameter is reached. The withdrawal rate and heating are then adjusted so as to maintain constant diameter until the process approaches an ending where again the rate and heating are increased so that the diameter decreases to form a cone portion at the end of the Czochralski rod.
In general, commercial methods using the Czochralski techniques are limited because of the amount of melt present in the crucible in the size and volume of the drawn silicon crystal.
At the melt temperature of silicon (about 1420.degree. C.), the surface of the silica (SiO.sub.2) crucible which is in contact with the melt dissolves. Some part of the dissolved silica evaporates from the surface of the melt as SiO (silicon monoxide). Another part of dissolved silica is incorporated into the growing crystal. The rest of the dissolved silica is retained in the molten silicon. Thus, the silica crucible which is used to contain the silicon melt is the source of oxygen which is found in silicon crystals grown by the conventional Czochralski technique.
Silicon crystals grown from melts contained in silica crucibles generally have a concentration of oxygen of about 10 to 50 parts per million atomic (ppma) as measured by ASTM standard F-121. The oxygen concentration in silicon crystals grown under Czochralski conditions prevalent in the industry today is not uniform but varies along the length of the crystal, for example, being higher at the seed end than in the middle and/or bottom or tang end of the crystal. In addition, there is a variation in oxygen concentration along the radius of a cross-sectional slice of the crystal. The non-uniform distribution of oxygen in silicon crystals grown by the conventional Czochralski technique using constant seed and crucible rotation rates is illustrated by the data in FIG. 1. FIG. 1 is a plot of the oxygen concentration measured at the center and edge of cross-sectional samples taken along the length of a 100 mm diameter silicon crystal. The non-uniform distribution of oxygen in this crystal is obvious. The oxygen concentration at the center of the crystal ranged from over 35 ppma at the seed end to less than 27 ppma near the tang end and a radial (center to edge) variation of over 13 ppma was found in parts of the crystal. Although these data pertain to a particular crystal, it is typical of crystals of other diameters grown under similar conditions.
Although oxygen in Czochralski grown silicon has long been regarded as a more or less undesirable impurity until recently, its presence has not been of great concern because the oxygen can be rendered electrically inactive by proper heat treatment. However, oxygen does tend to precipitate from solid solution or react with other impurities or lattice defects to form microdefects when the crystal is subjected to the diffusion and heat-treatment processes typical of modern electronic technology. Such microdefects can be either beneficial or deleterious to solid-state electronic device yields depending on the concentration of oxygen and the processing steps to which the crystal (or wafer) is subjected. Thus, to increase device yields, it is becoming ever more desirable and necessary to grow silicon crystals having a specified oxygen content and uniform distribution.
Methods for enhancing the concentration of oxygen in Czochralski grown silicon crystals have been described in the patent and other literature wherein, for example, during the Czochralski process the crucible rotation is stopped and started periodically during the process to provide fluid shearing at the melt-crucible interface. In another art taught method, oxygen content of Czochralski produced silicon is enhanced by changing surface characteristics of the portion of the silica vessel which is in contact with the melt so as to provide an increased oxygen concentration in the melt during the crystal drawing process. In yet another approach, lower oxygen concentrations are attempted through lower initial crystal rotation rates followed by relatively slow acceleration of the crystal rotation rate during the Czochralski pull.
All of the above methods have been evaluated and found to be lacking in certain respects. The range of oxygen concentrations obtainable from a particular process appear to be limited and/or fail to produce the desired uniform radial distribution of the oxygen concentration. The present invention allows for axial control of oxygen in Czochralski produced silicon rods over a wide range of concentration (from about 15 ppma to greater than 40 ppma) and results in significantly improved uniform axial and radial distributions of the oxygen.